Digital signals generate so-called "signal-eyes" of changing dimensions and positions which are separated by a region of intersymbol interference. Such signal-eyes are defined by the ensemble of all signal waveforms over the baud interval. For n signal levels, where n is an integer, n-1 vertically stacked signal-eyes are formed in the voltage-time domain. In an idealized digital transmission system, all signal transitions converge at an optimum sampling instant to points between the signal-eyes. These points will hereinafter be referred to as "points of convergence". In actual digital transmission systems, the signal distortions vary and are unpredictable. As a result, the signal transitions do not converge to points between the signal-eyes.
The use of digital transmission systems requires the ability to regenerate the transmitted signal after it has traveled through a noisy dispersive medium. Threshold circuits, which sense the amplitude of the digital signal with respect to a threshold, are used in the regeneration process. Preferably, the threshold circuit is adaptive, i.e., it automatically maintains the threshold at a constant position relative to the signal-eyes.
In one adaptive threshold circuit application, a comparator within the threshold circuit senses the digital signal with respect to a threshold passing through the signal-eyes. The output of the comparator is also coupled to feedback circuitry which automatically maintains the threshold at a predetermined position within the signal-eyes for minimum regeneration errors. This predetermined position is typically at the center of a signal-eye. Existing threshold circuitry of the type described suffers from several limitations. First, the feedback circuitry utilized requires precise analog reference signals. Second, the choice of the analog reference signal is affected by the degree of Nyquist filtering used to control the digital signal pulse shape.
In another signal regeneration application, threshold circuits are used to provide an error signal which drives adaptive transversal equalizers. This error signal is defined as the polarity of the difference between an actual digital signal and an idealized digital signal at the sampling instant. Circuits used in this second application suffer from the previously discussed limitations and, in addition, are not adaptive.